The present invention has been developed in connection with a conveyor train used to transport oil sand from a mine site to a hydrocarbon extraction plant. It will now be described in connection with that application, however the system can easily be adapted for use in other circumstances. It is to be understood, therefore that the invention is not to be considered limited solely to the application described herein.
Typically, a conveyor train system comprises a number of endless belt conveyors, hereinafter termed `conveyors`, serially arranged whereby the output load from one conveyor is transferred as the input load to the next conveyor in the train. Each conveyor, however, comprises a discrete unit.
The individual conveyor comprises lengths of flexible belts spliced together end to end to form the endless belt. In high capacity systems, each such belt is commonly formed of upper and lower rubber layers having reinforcing steel cords sandwiched therebetween. The belt is supported on a plurality of spaced, cushioned, anti-friction idlers. One or more driven pulleys is provided at the conveyor end for driving the belt. The driving pulleys are normally powered by conventional electrical motors. Additional drive motors may be included, if required to cope with variations in load. Tension means are incorporated to keep the belt taut.
For a system comprising belts of fixed length, it is common practice to run the conveyor train in a manner such that each conveyor draws the same percentage of its total rated power.
Where the load-carrying portion of any one belt can vary however, a major disadvantage in such a system is that overloading of a single conveyor can cause it to stall. When this occurs, the whole train must be brought to a halt. Restarting the train is difficult due to the great power needed to get the loaded conveyors under way again. Frequent stalling and restarting reduces productivity and increases equipment wear.
In applicants' commercial open-pit oil sand mining operation, carried out in the Fort McMurray region of Alberta, about 300,000 tonnes per day of oil sand is mined and conveyed to the extraction plant. This operation is illustrated schematically in FIG. 2. Mining is carried out using large draglines which excavate the oil sand to a depth in the order of 40 metres. The draglines deposit the oil sand in elongate windrows along the edge of the rectangular pit. A bucketwheel reclaimer transfers the oil sand from each windrow to a bridging conveyor, which feeds it onto the first conveyor of a train. As shown in FIG. 1, the train to the stacker may consist of 2 to 4 conveyors extending around the perimeter of the pit. In applicants' case, there are 4 draglines in use, each supplying a separate train. The trains all terminate at a common zone adjacent the extraction plant. Here the load from each train is transported by an inclined conveyor (or `stacker`) and deposited on an arcuate stack of oil sand. The extraction plant draws its feed from these 4 stacks.
It will be noted:
that the bucketwheel reclaimer works its way along the length of the windrow and thus the point, at which its bridging conveyor deposits the oil sand onto the first conveyor of the train, varies;
that the bucketwheel reclaimer deposits the oil sand in discrete spaced bucket loads on the transfer conveyor; and
that the various conveyors of a train vary in length. Exemplary lengths may range from 150 to 2500 meters.
Stated otherwise, the weight of the feed is not distributed uniformly along the conveyors and the loadbearing length of the conveyors in a train is also not uniform.
It is a requirement of the train concept that the velocity at which the various conveyors travel should be the same.So any resistance to motion will automatically require that the power drawn by the drive motors be increased, so as to maintain a constant belt speed. There exists a maximum safe level of electrical power that can be drawn from each motor without causing it to stall. One seeks always to optimize the loading of conveyors by running them at their maximum safe rated power draw. This is a power draw somewhat below the maximum (stall) power level. Maximum safe rated power correlates with the upper limit of the load that can be carried by the conveyor.
However, it is not difficult to inadvertently exceed this rating.
Conveyor power draw will fluctuate depending upon a number of factors. For example, the internal frictional losses associated with the conveyors are subject to numerous variations. Additionally, external factors such as fluctuations in ambient temperature or differences in the properties or grade of the oil sand per se will give rise to variations in power draw requirements. Expanding further on this latter point, the adhesive properties of the oil sand vary with its grade. If the oil sand is sticky, it will build up on the belt. This is turn affects the drag characteristics of the belt and hence the power draw. Or there may be density variations due to snow or rainfall pickup, which will alter the weight and adhesivity of the load. It is to be emphasized that the power draw fluctuations caused by the above factors are not insignificant.
However, the single predominant factor leading to rapid alternations in the power draw requirements resides in the inherent inconsistency and intermittent nature of the bucketwheel loading technique itself.
In using a bucketwheel reclaimer for loading the bridging or first conveyor of the train, the operator has only his experience and visual observation to guide him as to the rate at which the feed material should be deposited on the conveyor.
When the system was first put into use, the only method of control was full on/full off. An observer in the control tower at the systems delivery end simply radiosignalled that the bucketwheel reclaimer should start or stop adding feed to the first belt of the system.
In order to assist the reclaimer operator to more accurately gauge the optimum conveyor loading rate, a prior art control system was utilized. This method involved attaching wattage meters to the conveyor drive pulley motors and monitoring the power draw requirements thereof. An operator, located in a control tower positioned at the output end of the conveyor train monitored the wattage draw signals and instructed the reclaimer operator to either reduce or increase the feed rate depending upon whether an overload or reduced power draw was observed.
However, this prior art method allows of only crude control. The method fails to take into account the following:
The irregularity and intermittent nature of the rate of loading the feed from the individual buckets of the wheel;
the time lag between the actual loading of the feed material onto the belt, its transportation along the conveyor train, and the relaying of the feedback message to adjust the loading rate accordingly (this time lag may be of the order of twelve minutes);
the variation of effective length of the first conveyor actually in use. This length will change depending upon the position of the reclaimer as it moves along the windrow; and
the combined effects of irregular feed rate and power draw fluctuations on the optimum load each individual conveyor can carry.
It is to be noted that, in the present instance, the feed point of the system varies according to where, in the mine, the oil sand is loaded onto the system. As well, belt lengths are changed as the system is expanded to accommodate an increase in the size of the pit. The conveyor system is thus dynamic and variable. This differs from a static system where the feed point and belt lengths are fixed.
There exists, therefore, the need for a control system functional to adopt itself to systematic fluctuations and to optimize the load carried by the conveyor train whilst not exceeding permissible rated power draw for the drive motors.